Coating compositions containing dispersible powders typically must include a dispersing agent for such powders. It is essential that such dispersing agent be compatible both with any binder resin in the coating, and with the end use of the coated product. For example, magnetic recording tape is produced by coating a dispersion of magnetic particles in a binder solution onto a thermoplastic substrate, usually polyethylene terephthalate. The quality of the tape is determined by many factors, one of the most important of which is the degree to which the magnetic pigment has been uniformly dispersed.
It is well known that the binder of a typical magnetic tape comprises a combination of a soft, rubbery polymer, such as a thermoplastic polyurethane (e.g., Estane 5701F-1 from B. F. Goodrich) and a hard resin, such as a vinyl chloride/vinyl acetate/vinyl alcohol terpolymer (e.g., UCAR Solution Vinyl Resin VAGH from Union Carbide Corporation), in a weight ratio of about 30/70 to 70/30. Of these two types of polymers, the hard resin is typically a far better pigment-dispersing medium for the magnetic pigment, as judged by gloss measurements, microscopic observation and magnetic property evaluations.
Nevertheless, it has still been necessary to add low molecular weight, monomeric dispersing agents to formulations of magnetic pigment in solutions of VAGH in order adequately to disperse the pigment.
The need for these low molecular weight dispersants in order to achieve a high degree of dispersion is a serious problem since these materials are capable of exuding to the tape surface on aging, resulting in "dropouts," plasticization of the tape surface, fouling of the record/playback heads, and other problems.